The wild scene developed after a police officer at the Massachusetts Institute of Technology was shot and killed earlier Thursday night.

CNN early Friday was reporting that the FBI was on the scene in Watertown, Mass., and that two people have been arrested.

The Associated Press was reporting:

WATERTOWN, Mass. -- A tense night of police activity that left a university officer dead on campus just days after the Boston Marathon bombings and amid a hunt for two suspects caused officers to converge on a neighborhood outside Boston, where residents heard gunfire and explosions.

View full sizeOfficers wearing tactical gear arrive at the Watertown neighborhood of Boston. The Associated Press

The chaos in Watertown, about 10 miles west of Boston, occurred just hours after a Massachusetts Institute of Technology police officer was shot and killed on campus. It was unclear if the outbursts of violence were related. No arrests had been made in the officer's fatal shooting, and a manhunt was on for the shooter.

The officer had been responding to report of a disturbance Thursday night when he was shot multiple times, according to a statement from the Middlesex district attorney's office and Cambridge police. It said there were no other victims.

In Watertown, witnesses reported hearing multiple gunshots and explosions at about 1 a.m. Friday. Dozens of police officers and FBI agents were in the neighborhood and a helicopter circled overhead.

State police spokesman David Procopio said, "The incident in Watertown did involve what we believe to be explosive devices possibly, potentially, being used against the police officers."

Boston cab driver Imran Sais said he was standing on a street corner at a police barricade across from a diner when he heard an explosion.

"I heard a loud boom and then a rapid succession of pop, pop, pop," he said. "It sounded like automatic weapons. And then I heard the second explosion."

He said he could smell something burning and advanced to check it out but area residents at their windows yelled at him, "Hey, it's gunfire! Don't go that way!"

MIT said right after the 10:30 p.m. shooting that police were sweeping the campus in Cambridge and urged people to remain indoors. They urged people urged to stay away from the Stata Building, a mixed-use building with faculty offices, classrooms and a common area.

Hours later, MIT, which has about 11,000 students, said the campus was clear but the shooter was still on the loose.